1. Field of the Invention
The present invention relates to a paper machine, and, more particularly, to a method and apparatus of drying a structured fiber web on a structured fabric in a paper machine.
2. Description of the Related Art
In a wet molding process, a structured fabric in the standard Crescent Former press fabric position impresses a three dimensional surface on a web while the fibrous web is still wet. Such an invention is disclosed in International Publication No. WO 03/062528 A1. A suction box is disclosed for the purpose of shaping the fibrous web while wet to generate the three dimensional structure by removing air through the structural fabric. It is a physical displacement of portions of the fibrous web that leads to the three dimensional surface. Similar to the aforementioned method, a through air drying (TAD) technique is disclosed in U.S. Pat. No. 4,191,609. The TAD technique discloses how an already formed web is transferred and molded into an impression fabric. The transformation takes place on a web having a sheet solids level greater than 15%. This results in a low density pillow area in the fibrous web. These pillow areas are of a low basis weight since the already formed web is expanded to fill the valleys thereof. The impression of the fibrous web into a pattern, on an impression fabric, is carried out by passing a vacuum through the impression fabric to mold the fibrous web.
In a wet pressing operation a fibrous web sheet is compressed at a press nip to the point where hydraulic pressure drives water out of the fibrous web. It has been recognized that conventional wet pressing methods are inefficient in that only a small portion of a roller's circumference is used to process the paper web. To overcome this limitation, some attempts have been made to adapt a solid impermeable belt to form an extended nip for pressing the paper web to dewater the paper web. A problem with such an approach is that the impermeable belt prevents the flow of a drying fluid, such as air through the paper web. Extended nip press (ENP) belts are used throughout the paper industry as a way of increasing the actual pressing dwell time in a press nip. A shoe press is the apparatus that provides the ability of the ENP belt to have pressure applied therethrough, by having a stationary shoe that is configured to the curvature of the hard surface being pressed, for example, a solid press roll. In this way the nip can be extended well beyond the limit of the contact between the press rolls themselves. An ENP belt serves as a roll cover on the shoe press. This flexible belt is lubricated on the inside to prevent frictional damage. The belt and shoe press are non-permeable members and dewatering of the fibrous web is accomplished by the mechanical pressing thereof.
A fabric is utilized to carry the fiber web during the formation of the web. After the web takes form it is usually subjected to a drying process. The same fabric used during formation of the web or another fabric may come in contact with the web, to move the web across a vacuum section for the removal of moisture from the web. Additionally the web is sent, with a press fabric, through a press section. The problem is that if a structured fabric is sent to the press section no gain in dryness is achieved without using an expensive TAD method.
What is needed in the art is a method to effectively dewater a structured fibrous web.